This proposal outlines a comprehensive examination of the role of the lateral intra-parietal area (LIP) of parietal cortex in spatial attention. Clinical evidence implicates the parietal cortex in spatial attention; however, neurophysiological studies have not provided clear evidence for the role of parietal in attention. The majority of neurophysiological studies investigating attention provide no behavioral measure of attentional state. In the study proposed, attention is explicitly defined as the ability of an organism to alter visual processing efficiency at select spatial locations. A psychophysical methodology is presented, which enables us to re-direct attention to select spatial locations, and quantify attentional demand at a particular spatial location. Given that we can quantify an explicitly defined mental state, spatial attention, we will be able to determine to what extent changes in attentional state can be accounted for by underlying changes in the activity of neurons in area LIP. This research will be able to illuminate the role of area LIP in spatial attention and provide a framework for studies of attention in other brain areas.